Lithium primary batteries, which have high electromotive force and high energy density, have been widely used as the main power source or memory back-up power source for electronic devices, such as portable appliances and in-car electronic devices. Lithium primary batteries include: a positive electrode comprising a positive electrode active material that is, for example, a metal oxide such as manganese dioxide, or graphite fluoride; a separator; a negative electrode comprising lithium or a lithium alloy; and a non-aqueous electrolyte. Among lithium primary batteries, those using graphite fluoride have long shelf life and good stability in a high-temperature environment and can be used in a wide temperature range, compared with those using a metal oxide such as manganese dioxide.
With the recent trend toward smaller, lighter, and more sophisticated electronic devices, lithium primary batteries are also required to provide higher battery performance. In particular, when they are used as the main power source or memory back-up power source for in-car electronic devices, they are required to provide sufficient discharge characteristics in a wide temperature range from low temperature of approximately −40° C. to high temperature of approximately 125° C. Lithium primary batteries exhibit discharge characteristics of a voltage drop in the initial discharge stage followed by a gradual voltage rise. The greater the voltage drop in the initial discharge stage, the lower the battery performance. Such discharge characteristics are evident when a discharge is performed at a large current. Also, in some applications, primary batteries such as lithium primary batteries are repeatedly partially discharged (i.e., a part of the battery capacity is repeatedly discharged) until they are fully discharged.
To heighten the performance of lithium primary batteries, various attempts have been made, but improvements in the negative electrode are insufficient. Since lithium is highly reactive, a coating film containing various components is formed on the surface of a negative electrode comprising lithium or a lithium alloy during negative electrode production, battery fabrication and the like. Such coating film may determine the discharge characteristics of the battery. For example, when the battery is discharged in a low temperature environment, the coating film acts as a resistance component, thereby increasing the polarization (overvoltage) of the negative electrode in the initial discharge stage. As a result, the voltage in the initial discharge stage may significantly drop.
Also, in the case of using a positive electrode comprising graphite fluoride or a non-aqueous electrolyte including a fluorine-containing solute, fluorine derived therefrom reacts with lithium in the negative electrode surface to form a coating film of lithium fluoride on the negative electrode surface. Since lithium fluoride is an insulator, the coating film may significantly increase the negative electrode polarization during discharge. In particular, in a low temperature environment of 0° C. or less, the negative electrode polarization significantly increases in the initial stage of a large-current discharge, so that the voltage drop in the initial discharge stage becomes evident. To suppress the voltage drop in the initial discharge stage, it is necessary to reduce the negative electrode polarization caused by the coating film formed on the negative electrode surface, and the like.
As described above, since the coating film formed on the lithium-containing negative electrode surface has a large impact on battery performance, various proposals have been made.
For example, when lithium is present on the surface of a negative electrode, the lithium is oxidized, so that an oxide coating film of lithium oxide or lithium hydroxide is formed on the surface. The oxide coating film increases the internal impedance of the battery, lowers the discharge performance of the battery, and increases the variation in battery performance. Noting this, Japanese Laid-Open Patent Publication No. 2005-216601 (hereinafter “Patent Document 1”) has made a proposal. Patent Document 1 relates to a negative electrode including a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector, wherein a coating film containing lithium carbonate (hereinafter “lithium carbonate coating film”) is formed on the surface of the negative electrode active material layer.
The technique of Patent Document 1 is characterized in that when a negative electrode having lithium on the surface is used in a lithium ion secondary battery, a lithium carbonate coating film is formed on the surface of the negative electrode. The negative electrode used therein includes a negative electrode active material layer and a negative electrode current collector, wherein lithium is absorbed in the negative electrode active material layer. That is, Patent Document 1 intends to suppress the formation of an oxide coating film on the negative electrode surface by forming a lithium carbonate coating film on the negative electrode surface. In Patent Document 1, the lithium carbonate coating film is formed by causing the negative electrode active material layer to absorb lithium and bringing it into contact with carbon dioxide.
The technique of Patent Document 1 is effective when using a positive electrode comprising a metal oxide. However, when using a non-aqueous electrolyte including a fluorine-containing solute or a positive electrode comprising graphite fluoride, even if the lithium carbonate coating film is formed on the surface of the negative electrode active material layer, lithium fluoride, which is an insulator, is inevitably formed. As mentioned above, lithium fluoride significantly increases the negative electrode polarization and hence voltage drop in the initial discharge stage. As such, the lithium carbonate coating film is unable to prevent the formation of lithium fluoride.
Also, Japanese Laid-Open Patent Publication No. 2006-236890 (hereinafter “Patent Document 2”) has made a proposal noting that the lithium carbonate coating film formed on the negative electrode surface is destroyed by battery partial discharge, so that a coating film different from the lithium carbonate coating film is formed on the negative electrode surface, thereby promoting the occurrence of a voltage drop. Patent Document 2 relates to a lithium primary battery including a positive electrode containing carbon fluoride, a negative electrode containing lithium, and a non-aqueous electrolyte. The battery of Patent Document 2 is characterized by the negative electrode and the non-aqueous electrolyte. The negative electrode has a lithium carbonate coating film with a thickness of 10 nm or more on the surface. The non-aqueous electrolyte contains 1,2-dimethoxyethane as the non-aqueous solvent, and the moisture content is 100 to 200 ppm.
In Patent Document 2, the use of the specific non-aqueous electrolyte stabilizes the impedance of the battery during storage after partial discharge, and suppresses the destruction of the lithium carbonate coating film due to partial discharge of the battery. However, moisture contained in the non-aqueous electrolyte reacts with the non-aqueous electrolyte, and the reaction products adhere to the negative electrode surface, thereby increasing the negative electrode polarization during partial discharge. Therefore, although the lithium primary battery of Patent Document 2 has high performance and good practicability, it needs to be improved in terms of further reducing the voltage drop during partial discharge.
Further, Japanese Laid-Open Patent Publication No. 2006-339046 (hereinafter “Patent Document 3”) has made a proposal noting that lithium in the negative electrode surface reacts with components contained in the non-aqueous electrolyte to form an electrically inactive lithium compound on the negative electrode surface, which increases the negative electrode polarization and makes the voltage drop during discharge significant. Patent Document 3 relates to a lithium primary battery including a positive electrode, a negative electrode comprising lithium or a lithium alloy, a separator, and a non-aqueous electrolyte, wherein a carbon black layer is formed on the negative electrode surface. The carbon black layer formed on the negative electrode surface suppresses the reaction between the lithium and the non-aqueous electrolyte. It therefore suppresses an increase of a lithium compound which is an electrically inactive resistor on the negative electrode surface. As a result, it is possible to obtain a very high performance lithium primary battery that exhibits little voltage drop in the initial stage of a large-current discharge in a low temperature environment and after high temperature storage.
When the carbon black layer formed on the negative electrode surface comes into contact with the electrolyte, the potential of the carbon black becomes almost equivalent to that of the lithium or lithium alloy. Thus, a reaction of lithium ion insertion into the carbon black particles and a reaction of electrolyte decomposition proceed, so that decomposition products are deposited on the negative electrode surface in a short period of time. The decomposition products form a coating film that protects the negative electrode surface, thereby forming a stable negative electrode/electrolyte interface, compared with when carbon black is not used. Patent Document 3 states that when such interface is formed, negative electrode polarization in a low temperature environment and an increase in battery internal resistance due to high temperature storage are suppressed.
However, since a coating film containing various components is formed on the lithium surface, the reaction of lithium ion insertion into the carbon black particles and the electrolyte decomposition reaction proceed unevenly. That is, the electrolyte decomposition reaction may not proceed sufficiently. Hence, compared with the case of not using carbon black, negative electrode polarization in a low temperature environment and an increase in battery internal resistance due to high temperature storage are suppressed, but the effect of such suppression may be insufficient.